1. Field of the Invention
The present invention relates to a polyimide-based macromolecular compound which is produced by using DOCDA and one or more diamines selected from the group consisting of MDA, ODA, PDA, TDA, TrMPD, TeMPD and MBCA; and a gas-separation asymmetrical hollow-fibre membrane comprising the same. More particularly, the present invention relates to a polyimide-based macromolecular compound which is advantageously used for the production of a hollow-fibre membrane having the asymmetrical structure since the compound not only has excellent gas permeability and high selectivity but also has high heat resistance, high chemical resistance, and excellent mechanical properties, along with outstanding solubility in polar organic solvents and high molecular weight which is appropriate for dry jet-wet spinning.
2. Description of the Related Art
The conventional process for gas-separation widely used in the industrial field is exemplified by the energy-guzzling distillation, extraction, evaporation, absorption, adsorption, cryogenics, and crystallization. Such separation process uses approximately 40% of total energy consumed in the industrial site. Studies and many attempts have been made for establishing a separation process that can replace the conventional energy consuming process. And as a result, a membrane separation process using a membrane for separation is rising as an alternative.
The membrane separation process using a separation membrane is known not to accompany phase change in the course of separation, suggesting that it consumes less energy, is pro-environmental, and does not require a huge facility owing to the simple process with simple equipments. That is, the operation, maintenance, and management, of the method is easy and advantageous. Further the above method allows the scale-up easily and can also be operated in hybrid system with other separation processes.
In the history of study about such a separation membrane, the full-scale experiment on a gas-separation membrane has started in 1960s, and a preparation method of an asymmetrical reverse osmosis membrane established by Loeb and Sourirajan has been applied to gas-separation in 1970s, since then the studies on the separation membrane have advanced rapidly.
In the early 1977, Monsanto Co. first attempted to apply a hollow-fibre membrane to the process of hydrogen separation and collection that is generated in the course of purification, which was quite successful. Based on the successful attempt, the company began to sale the membrane in the brand name of PRISM in 1979. So, the company Monsanto now sales 75 membrane separation systems world-widely.
In addition to the PRISM provided by Monsanto, a spiral wound CA module provided by Separex has been successfully applied to the processes of recovering H2 from the waste gas generated in the course of refinery process and petrochemical process and regulating H2 composition at ammonia plant and also applied to the process running at methanol plant.
An attempt has been made to apply the separation membrane further to the separation of CO2 in addition to the separation of H2. In particular, natural gas collected from oil field is generally composed of 40˜45 mol % CO2 and 54˜59 mol % CH4. The membrane separation process was successfully introduced for the separation of CO2/CH4.
Studies followed, and as a result the process of purifying CH4 from bio-gas or natural gas by eliminating CO2 has been commercialized, which triggered the foundation of many companies. The method keeps advancing to broaden its application and now can compete with the conventional methods such as cryogenics, absorption, and adsorption well enough to replace them.
Considering economics of the gas separation process using a membrane, the selectivity of membrane material influences the recovery rate, and the permeability decides the cost, of membrane module and plant. So, even after the commercialization of gas-separation membrane, additional studies are under-going to develop a separation membrane that has high permeability and selectivity and is applicable under harsh condition such as high temperature and high pressure.
The material used for the production of a membrane for gas separation is mostly a nonporous macromolecular membrane. To use the separation membrane made of a macromolecular material in the industry, the membrane needs to have high gas permeability and selectivity and at the same time has to have excellent thermo-stability and mechanical stability so as to endure high pressure and high temperature when it turns into a thin membrane. In addition, the membrane has to have chemical stability against gas toxicity. In the case of mass-production of such a membrane in the form of a hollow-fibre membrane, the solubility in organic solvents applicable for phase inversion process and high molecular weight for high viscosity appropriate for spinning are required.
At least 1,000 materials have been studied, and tried so far for the separation membrane. However, only 8˜9 materials have been successfully commercialized considering the price, the productivity, and the properties of the material. The commercialized gas separation membrane materials so far are exemplified by polysulfone, polyimide, cellulose acetate, polycarbonate, and polypyrrolidone. Polysulfone is commercialized by Air Products, Co., USA, and the brominated polycarbonate is commercialized by MGI. Polyimide is commercialized by Ube, Japan, and cellulose acetate is commercialized by Dow.
Among them, polyimide is characterized by high chemical stability and the thermo-stability, compared with such materials as polysulfone, cellulose acetate and polycarbonate. Besides, polyimide has polarity and high capability of gas separation particularly carbon dioxide/methane, oxygen/nitrogen, hydrogen/nitrogen, and carbon dioxide/nitrogen, owing to the stiff molecular chain thereof. Because of such properties of polyimide, it has been a promising target of many researches to develop a separation membrane. For example, Ube Co., Japan commercialized a gas separation hollow-fibre membrane using the self-developed biphenyl aromatic polyimide Upilex-R (BPDA-ODA) copolymer. Evonic Co., Germany, was also succeeded in the commercialization of a hollow-fibre membrane for methane separation using the aromatic polyimide copolymer P84. Air Liguide Co., USA, developed and commercialized a gas separation hollow-fiber membrane with the soluble polyimide Matrimide using the alicycle dianhydride produced by Hunsmann Co., USA, and now not only sells it in the market but also extends its market further by applying thereof to the fields of bio-methane and natural gas. Such companies developed their own separation membranes using the self-developed polyimide material and supplied them to the world-market exclusively.
The applicable range of such separation membrane has been rapidly widened because of world-wide oil depletion, which includes the purification of natural gas, the purification of bio-methane generated from organic waste, the membrane separation for recovering CO2 from combustion flue gas, and the processes required in the field of petrochemistry using a membrane for the separation of H2/CO2 and N2/O2 in the combined cycle power plant or oxy-fuel combustion process. The market of a separation membrane is also growing to the field of separation of H2 in the course of synthetic gas production. To develop a highly competitive separation membrane in the field of purification of methane/CO2, O2/N2, and H2/CO2, a novel polyimide material usable for the preparation of a hollow-fibre membrane that has high solubility in organic solvents and high molecular weight is urgently required.
U.S. Pat. No. 4,851,505 and U.S. Pat. No. 4,912,197 describe the polyimide gas-separation membrane characterized by high selectivity, productivity, and mechanical stability using the annealed polyimide polymer having a specific repeat unit in the course of the process and excellent solubility in general solvents, which is therefore advantageous in reducing complexity in the polymer processing.
US Patent Publication no. 2009-0227744 describes the membrane displaying high selectivity, productivity, and mechanical stability by using the annealed polyimide polymer having a specific repeat unit.
Therefore, the present inventors prepared a DOCDA polyimide macromolecular compound with changing diamine using DOCDA (5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride) known as one of polyimide materials for gas-separation that has excellent solubility in organic solvents owing to its alicyclic ring structure which is advantageous for making torsion or asymmetry in the main chain as a monomer. Based on that, the present inventors further studied and established an asymmetrical gas-separation hollow-fibre membrane by dry jet-wet spinning, and confirmed that the prepared membrane had excellent gas-separation characteristics, leading to the completion of this invention.